This invention relates to a crystalline silicoaluminophosphate molecular sieve and to its synthesis. It more particularly relates to the synthesis of a crystalline silicoaluminophosphate molecular sieve which has a bulk P.sub.2 O.sub.5 /Al.sub.2 O.sub.3 mole ratio and bulk SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio different from their corresponding surface P.sub.2 O.sub.5 /Al.sub.2 O.sub.3 and surface SiO.sub.2 /Al.sub.2 O.sub.3 mole ratios.
Silicoaluminophosphates are taught in U.S. Pat. No. 4,440,871, for example. Silicoaluminophosphate materials are both microporous and crystalline and have a three-dimensional crystal framework of PO.sub.2.sup.+, AlO.sub.2.sup.- and SiO.sub.2 tetrahedral units and, exclusive of any alkali metal or other cation which may optionally be present, an as-synthesized empirical chemical composition on an anhydrous basis of: EQU mR:(Si.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m"represents the moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from 0 to 0.3, the maximum value in each case depending upon the molecular dimensions of the templating agent and the available void volume of the pore system of the particular silicoaluminophosphate species involved; "x", "y", and "z" represent the mole fractions of silicon, aluminum, and phosphorus, respectively, present as tetrahedral oxides. The minimum value for each "x", "y", and "z" is 0.01 and preferably 0.02. The maximum value for "x" is 0.98; for "y" is 0.60; and for "z" is 0.52.
It is disclosed in U.S. Patent No. 4,440,871 that while it is not essential to the synthesis of SAPO compositions, it has been found that in general, stirring or other moderate agitation of the reaction mixture and/or seeding the reaction mixture with seed crystals of either the SAPO species to be produced or a topologically similar aluminophosphate or aluminosilicate composition, facilitates the crystallization procedure. These silicoaluminophosphates exhibit several physical and chemical properties which are characteristic of aluminosilicate zeolites and aluminophosphates.
Silicoaluminophosphate SAPO-11 and its conventional method of preparation are taught by U.S. Pat. No. 4,440,871.